1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which an image is formed on a recording medium by ejecting liquid from ejection ports toward a recording medium such as paper by changing the volume of pressure chambers connected to the ejection ports.
2. Description of the Related Art
There is known an image forming apparatus in the related art, which forms an image on a recording medium, such as paper, by ejecting ink from a plurality of nozzles toward the recording medium, while moving a liquid ejection head having an arrangement of the nozzles and the recording medium, relatively with respect to each other.
A known liquid ejection head mounted in the image forming apparatus is, for example, a piezo-type liquid ejection head, in which ink is supplied to pressure chambers connected to nozzles, and the volume of the pressure chambers is changed, thereby causing the ink inside the pressure chambers to be ejected from the nozzles, by applying a drive signal corresponding to the image data to piezoelectric elements that are installed through a diaphragm plate on the outer side of the pressure chambers.
In the image forming apparatus having the inkjet head, in order to be able to form an image at high speed by shortening the ink ejection cycle, it is necessary to improve the refill speed of the liquid into the pressure chambers.
Japanese Patent Application Publication No. 4-45947 (and in particular, FIGS. 1, 2 and 3) discloses that the inkjet head is provided with drive elements (individual drive piezoelectric elements) for changing the volume of the pressure chambers, and another drive element (common liquid chamber driving piezoelectric element) for changing the volume of the common liquid chamber which supplies liquid to the pressure chambers, in such a manner that a pulse waveform is applied to the common liquid chamber driving piezoelectric element immediately after the meniscus cut-off time.
However, there is difficulty in refilling ink without causing disturbance to ink ejection.
More specifically, if a pulse waveform is applied to the common liquid chamber driving piezoelectric element immediately after the meniscus cut-off time, as in Japanese Patent Application Publication No. 4-45947, then as shown in FIG. 9A, immediately after the meniscus cut-off time there is a state in which the desired ink droplet 82 has separated and left a liquid column 81 that still projects from the nozzle 51, and if the common liquid chamber is pressurized in this state, then as shown in FIG. 9B, there is probability that a relatively small ink droplet 89 (a so-called “satellite ink droplet”) is also propelled from the remaining liquid column 81 in addition to the desired ink droplet 82 that has been ejected previously. In general, it is difficult to apply a pulse waveform to the common liquid chamber driving piezoelectric element at a level that does not cause ink to be ejected from the nozzles, and consequently, this may cause detrimental effects to image quality.
Furthermore, there is a difficulty in adjusting the drive timings of the ink ejection and the ink refilling.
For example, in the inkjet head disclosed in Japanese Patent Application Publication No. 4-45947, it is necessary to apply a pulse waveform to the common liquid chamber driving piezoelectric element momentarily, immediately after the meniscus cut-off time, and in order to apply the pulse waveform to the common liquid chamber driving piezoelectric element at this strict timing, generally, it is necessary to adjust the timing at each ejection operation.